Definition of Mora

Mora or "morae" (as it's called in its plural form) is a writing device commonly used in poetry. Historically, it was used extensively in poetry, especially Haiku. However, today it is being used less and less. Morae are extremely difficult to understand in English and, as such, are less likely to be used. In Japanese, for example, Morae are referred to as "on" and are easy to spot and easy to use. It's as commonly known to Japanese as syllables are in English.

Nonetheless, to reach a full understanding of haiku and other non-English poetry types, it's important to know how a mora is defined and how it is used.

Definition:

Mora

n., pl. morae or moras. The minimal unit used in phonology that determines stress and timing. It is equal to the short syllable.

History and Meaning

The word mora comes from the Latin word for "linger, delay", which was also used to translate the Greek word chronos (time) in its metrical sense.

A syllable that contains one mora is said to be monomoraic; one with two mora is called bimoraic. Monomoraic syllables are said to be light syllables, while bimoraic syllables are said to be heavy syllables, and trimoraic syllables are said to be superheavy syllables. There is no language that uses syllables containing four or more morae.

In general, moras are formed as follows:

The syllable onset (the first consonant(s) of the syllable) does not represent any mora. For example, in the word "jump" 'j' does not count towards a mora—the entire word has two morae.

The syllable nucleus represents one mora in the case of a short vowel, and two moras in the case of a long vowel or diphthong. Consonants serving as syllable nuclei also represent one mora if short and two if long. (Slovak is an example of a language that has both long and short consonantal nuclei.) For example, in the word "cake" the a is long, so it is two morae—the entire word contains three morae.

In some languages (for example, Japanese), the coda represents one mora, and in others (for example, Irish) it does not. In English, it is clear that the codas of stressed syllables represent a mora (thus, the word cat is bimoraic), but it is not clear whether the codas of unstressed syllables do (the second syllable of the word rabbit might be monomoraic).
In some languages, a syllable with a long vowel or diphthong in the nucleus and one or more consonants in the coda is said to be trimoraic.